Cannabis sativa L. (cannabis, hemp, marijuana) is one of the oldest and most versatile domesticated plants, which today finds use as source of medicinal, food, cosmetic and industrial products. It is also well known for its use as an illicit drug owing to its content of psychoactive cannabinoids (e.g. Δ9-tetrahydrocannabinol, Δ9-THC). Cannabinoids and other drugs that act through mammalian cannabinoid receptors are being explored for the treatment of diverse conditions such as chronic pain, multiple sclerosis and epilepsy.
Cannabinoids have their biosynthetic origins in both polyketide and terpenoid metabolism and are termed terpenophenolics or prenylated polyketides (Page J., Nagel J. (2006) Biosynthesis of terpenophenolics in hop and cannabis. In J T Romeo, ed, Integrative Plant Biochemistry, Vol. 40. Elsevier, Oxford, pp 179-210.). Cannabinoid biosynthesis occurs primarily in glandular trichomes that cover female flowers at a high density. Cannabinoids are formed by a three-step biosynthetic process: polyketide formation, aromatic prenylation and cyclization (see FIG. 1).
The first enzymatic step in cannabinoid biosynthesis is the formation of olivetolic acid by a putative polyketide synthase enzyme that catalyzes the condensation of hexanoyl coenzyme A (CoA) and malonyl CoA. A Type III polyketide synthase, termed “olivetol synthase” and referred to herein as polyketide synthase/olivetol synthase (CsPKS/olivetol synthase), from Cannabis sativa has recently been shown to form olivetol and several pyrone products but not olivetolic acid (Taura F, Tanaka S, Taguchi C, Fukamizu T, Tanaka H, Shoyama Y, Morimoto, S. (2009) Characterization of olivetol synthase, Type III a polyketide synthase putatively involved in cannabinoid biosynthetic pathway. FEBS Lett. 583: 2061-2066.). The nucleotide sequence of the gene encoding CsPKS/olivetol synthase is found in GenBank under accession number AB164375 with the polypeptide as accession BAG14339. The aforementioned products include the pyrones hexanoytriacetic lactone (HTAL) and pentyldiacetic lactone (PDAL). The reason for the inability of this enzyme to form olivetolic acid, which is clearly a pathway intermediate based on the carboxylate structure of the cannabinoids, is not known. The lack of olivetolic acid formation by this polyketide synthase from cannabis was confirmed by the inventors, as further described herein and also by Marks et al. (Marks M D, Tian L, Wenger J P, Omburo S N, Soto-Fuentes W, He J, Gang D R, Weiblen G D, Dixon R A. (2009) Identification of candidate genes affecting Delta9-tetrahydrocannabinol biosynthesis in Cannabis sativa. J Exp Bot. 60, 3715-3726.).
The second enzymatic step is the prenylation of olivetolic acid to form cannabigerolic acid (CBGA) by the enzyme geranylpyrophosphate:olivetolate geranyltransferase. This enzyme is an aromatic prenyltransferase and is the subject of commonly owned copending U.S. Provisional patent applications U.S. Ser. No. 61/272,057 filed Aug. 12, 2009 and U.S. Ser. No. 61/272,117 filed Aug. 18, 2009. CBGA is a central branch-point intermediate for the biosynthesis of the different classes of cannabinoids. Cyclization of CBGA yields Δ9-tetrahydrocannabinolic acid (THCA) or its isomers cannabidiolic acid (CBDA) or cannabichromenic acid (CBCA) (see FIG. 1). The Shoyama group has previously published the identification and purification of the three enzymes responsible for these cyclizations (Morimoto S, Komatsu K, Taura F, Shoyama, Y. (1998) Purification and characterization of cannabichromenic acid synthase from Cannabis sativa. Phytochemistry. 49: 1525-1529; Taura F, Morimoto S, Shoyama Y. (1996) Purification and characterization of cannabidiolic-acid synthase from Cannabis sativa L. Biochemical analysis of a novel enzyme that catalyzes the oxidocyclization of cannabigerolic acid to cannabidiolic acid. J Biol Chem. 271: 17411-17416; and Taura F, Morimoto S, Shoyama Y, Mechoulam R. (1995) First direct evidence for the mechanism of 1-tetrahydrocannabinolic acid biosynthesis. J Am Chem Soc. 117: 9766-9767). Cloning of THCA and CBDA synthases has also been previously published (Sirikantaramas S, Taura F, Tanaka Y, Ishikawa Y, Morimoto S, Shoyama Y. (2005) Tetrahydrocannabinolic acid synthase, the enzyme controlling marijuana psychoactivity, is secreted into the storage cavity of the glandular trichomes. Plant Cell Physiol. 46: 1578-1582.; Taura F, Sirikantaramas S, Shoyama Y, Yoshikai K, Shoyama Y, Morimoto S. (2007) Cannabidiolic-acid synthase, the chemotype-determining enzyme in the fiber-type Cannabis sativa. FEBS Lett. 581: 2929-2934. The genes for THCA synthase and CBDA synthase have been reported in Japan (Japanese Patent Publication 2000-078979; Japanese Patent Publication 2001-029082).
Cannabinoids are valuable plant-derived natural products. Genes encoding enzymes involved in cannabinoid biosynthesis will be useful in metabolic engineering of cannabis varieties that contain ultra low levels of THC and other cannabinoids via targeted mutagenesis (e.g. TILLING) or other gene knockout techniques. Such genes may also prove useful for creation of specific cannabis varieties for the production of cannabinoid-based pharmaceuticals, or for reconstituting cannabinoid biosynthesis in heterologous organisms such as bacteria or yeast, or for producing cannabinoids in cell-free systems that utilize recombinant proteins.
Genes encoding enzymes of cannabinoid biosynthesis can also be useful in synthesis of cannabinoid analogs and synthesis of analogs of cannabinoid precursors. Cannabinoid analogs have been previously synthesized and may be useful as pharmaceutical products.
There remains a need in the art to identify enzymes, and nucleotide sequences encoding such enzymes, that are involved in the synthesis of aromatic polyketides.